Disk arrays comprising a multiplicity of small inexpensive disk drives, such as the 51/4 or 31/2 inch disk drives currently used in personal computers and workstations, connected in parallel have emerged as a low cost alternative to the use of single large disks for non-volatile storage of information within a computer system. The disk array appears as a single large fast disk to the host system but offers improvements in performance, reliability, power consumption and scalability over a single large magnetic disk. Several disk array alternatives are discussed in an article titled "A Case for Redundant Arrays of Inexpensive Disks (RAID)" by David A. Patterson, Garth Gibson and Randy H. Katz; University of California Report No. UCB/CSD 87/391, Dec. 1987. The article, incorporated herein by reference, discusses disk arrays and the improvements in performance, reliability, power consumption and scalability that disk arrays provide in comparison to single large magnetic disks.
A disk array supporting RAID levels with redundant data must sustain a single disk failure or system fault with high reliability and availability. This means the integrity of the data blocks and parity blocks across the array must be maintained. There are several windows of vulnerability which must be protected or data integrity cannot be assured. These windows occur when there is a disk drive or system failure during I/O operations on the array.